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Full text of "Treatise On Analysis Vol-Ii"

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(15.11 .7) For N to be self-adjoint (resp. unitary) it is necessary and sufficient
that Sp(N) c R (resp. Sp(JV) c:U).IfHis self-adjoint, then

inf(SpCff)) =   inf (H • x \ x),

(                                                  II*""1

sup(Sp(#)) = sup (H-x|x),


(                         ||H||=   sup|(H-x|x)|.


In the first assertion, we have already seen (15.4.12) that the conditions are
necessary. To show that they are sufficient, it is enough (by virtue of (1 5.1 1 .5))
to prove them for the Nn\ and this is immediately done, because, when Nn
is identified with multiplication by the class of lc in L^(Sp(Nt)9 /4n), the
operator N* is identified with multiplication by the class of the function
Ł*-->Ł• To prove (, it is enough to show that, for a self-adjoint
operator H to be such that (H • x \ x) ^ 0 for all x e E (in which case we say
that H is positive and we write H ^ 0), it is necessary and sufficient that
Sp(/f) c R+ . In view of (15.11.5) and the relation

(                 (H-xIx^KH.-xJxJ


(with notation analogous to that of (15.11.3)) we are reduced to proving the
assertion for simple self-adjoint operators H* . If we identify Hn with multipli-
cation by the class of lc in Lc(Sp(Hn), /*„), what we have to prove is that

Sp(Hn) <= R+ if and only if JV(0 4UŁ) ^ 0 for every function /Ł 0 in
^rc(Sp(/fn)). Now, if M is the intersection of Sp(Hn) with the complement
] — oo, 0[ of R+ in R, then the relation M ^0 would imply /JB(M) > 0
(15.1.14). Since ] — oo.0[ is the union of the intervals ] — oo, — 1/Ť], there
would exist m > 0 such that

and consequently f C^CO 4*n(C) ^ -a/w < 0, contrary to hypothesis.
Finally, the relation ( follows from ( and (15.4.14,1),
because the spectral radius of H is equal to the larger of |inf (Sp(^T))|,

(15.11.8) (i) For each function fe ^rc(Sp(N)), the spectrum of f(N) is
contained in f(Sp(N)) (closure in C), and

(                        ||/(N)|| g sup   |/(C)|.ntegers n such that a